Aluminum foil is commonly used as an electrode material for aluminum electrolytic capacitors. The surface area of an aluminum foil can generally be increased by performing an etching treatment to form etching pits. The etched surface of the electrode material is then anodized to form an oxide film thereon, which functions as a dielectric. By etching the aluminum foil and applying one of various voltages to the surface thereof to match the voltage that is to be used, an anodic oxide film can be formed, thus enabling various aluminum anodes (foils) to be produced for electrolytic capacitors that are suited to specific applications.
However, etching treatments require the use of an aqueous hydrochloric acid solution that contains sulfuric acid, phosphoric acid, nitric acid, etc., in hydrochloric acid. Hydrochloric acid has a considerable environmental impact, and its disposal is also a burden from the viewpoint of the production process and economically. In the etching treatment, non-uniform etching pits may occur, pits may be easily united in some regions, and pits may be difficult to form in other regions, posing problems in pit control. Furthermore, the formation of many small pits may lower the hardness of an electrode material.
Therefore, the development of a novel method for increasing the surface area of an aluminum foil, which does not require etching, has been in demand. For example, Patent Literature 1 proposes a method that increases the surface area by adhering fine aluminum powder onto the surface of the aluminum foil by a vapor deposition method and then sintering. Patent Literature 2 proposes a method that increases the surface area by stacking aluminum particles while maintaining a space between each particle and sintering the particles. It was confirmed that this method makes it possible to obtain a surface area greater than the area of pits formed by etching.
However, the vapor deposition method disclosed in Patent Literature 1 has difficulty in adhering fine aluminum powder thickly; therefore, there is a limit to the increase in electrostatic capacitance. It is also difficult to deposit fine aluminum particles with sufficient space therebetween; therefore, this method is not suitable for producing an electrode material for medium- to high-voltage aluminum electrolytic capacitors.
In a mode using the sintered body disclosed in Patent Literature 2, it is necessary to make the sintered body thick in order to increase the electrostatic capacitance. However, when the sintered body is made thick, the bending strength undesirably lowers. Having a low bending strength causes the electrode material to break when it is wound to form a capacitor element. In particular, when the bending number (=the number of bends capable of withstanding breaking) is 0, the material will not endure processing in an actual chemical conversion line, thus lowering the mass production capability of the electrode material.